Joining between metallic components is currently approached in a number of ways, each with its own limitations.
The use of fasteners is commonplace but leads to increased component weight due to the need for bolting flanges. Holes drilled to enable bolting also create stress concentrations which can act as crack initiation sites. As such, fastened joints are not well suited to many aerospace applications.
A common method of joining dissimilar metallic materials is by welding or brazing. However, these processes have their limitations. Welding can only be performed on certain materials and material combinations, and the welding process can introduce local heat affected zones which can be brittle and may adversely affect fatigue performance.
Adhesive joints between metallic parts are possible. However, their weakness in peel and in tension makes them limited in their suitability for use within conventional aerospace structures. Adhesives also tend to be sensitive to moisture and elevated temperatures, making them unsuitable for many applications.
WO 2008/110835 A1 describes a method by which surface projections are “grown” on a bond surface of a metallic component in a series of layers by an additive fabrication process. A pair of metallic components having complimentary surface projections may be brought together and bonded using a thin layer of adhesive. The complimentary projections improves the surface area of the bond.